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.\" FIXME Something could be added to this page (or exit(2))
.\" about exit_robust_list processing
.\"
.TH GET_ROBUST_LIST 2 2016-07-17 Linux "Linux System Calls"
.SH NAME
get_robust_list, set_robust_list \- get/set list of robust futexes
.SH SYNOPSIS
.nf
.B #include <linux/futex.h>
.B #include <sys/types.h>
.B #include <syscall.h>
.sp
.BI "long get_robust_list(int " pid ", struct robust_list_head **" head_ptr ,
.BI "                     size_t *" len_ptr );
.BI "long set_robust_list(struct robust_list_head *" head ", size_t " len );
.fi

.IR Note :
There are no glibc wrappers for these system calls; see NOTES.
.SH DESCRIPTION
The robust futex implementation needs to maintain per-thread lists of
the robust futexes which are to be unlocked when the thread exits.
These lists are managed in user space; the kernel is notified about only
the location of the head of the list.

The
.BR get_robust_list ()
system call returns the head of the robust futex list of the thread
whose thread ID is specified in
.IR pid .
If
.I pid
is 0,
the head of the list for the calling thread is returned.
The list head is stored in the location pointed to by
.IR head_ptr .
The size of the object pointed to by
.I **head_ptr
is stored in
.IR len_ptr .

Permission to employ
.BR get_robust_list ()
is governed by a ptrace access mode
.B PTRACE_MODE_READ_REALCREDS
check; see
.BR ptrace (2).

The
.BR set_robust_list ()
system call requests the kernel to record the head of the list of
robust futexes owned by the calling thread.
The
.I head
argument is the list head to record.
The
.I len
argument should be
.IR sizeof(*head) .
.SH RETURN VALUE
The
.BR set_robust_list ()
and
.BR get_robust_list ()
system calls return zero when the operation is successful,
an error code otherwise.
.SH ERRORS
The
.BR set_robust_list ()
system call can fail with the following error:
.TP
.B EINVAL
.I len
does not equal
.IR "sizeof(struct\ robust_list_head)" .
.PP
The
.BR get_robust_list ()
system call can fail with the following errors:
.TP
.B EPERM
The calling process does not have permission to see the robust futex list of
the thread with the thread ID
.IR pid ,
and does not have the
.BR CAP_SYS_PTRACE
capability.
.TP
.B ESRCH
No thread with the thread ID
.I pid
could be found.
.TP
.B EFAULT
The head of the robust futex list can't be stored at the location
.IR head .
.SH VERSIONS
These system calls were added in Linux 2.6.17.
.SH NOTES
These system calls are not needed by normal applications.
No support for them is provided in glibc.
In the unlikely event that you want to call them directly, use
.BR syscall (2).

A thread can have only one robust futex list;
therefore applications that wish
to use this functionality should use the robust mutexes provided by glibc.
.SH SEE ALSO
.BR futex (2)
.\" .BR pthread_mutexattr_setrobust_np (3)

.IR Documentation/robust-futexes.txt
and
.IR Documentation/robust-futex-ABI.txt
in the Linux kernel source tree
.\" http://lwn.net/Articles/172149/
